Introduction Left bundle branch block (LBBB) is prevalent in 0.2%–1.1% of the general population and has been shown to carry an increased risk of future cardiac disease as well as mortality. Dependant on symptoms, 2D-echocardiography is likely to be first line of investigation. A normal echocardiogram in this context raises a dilemma as to whether to investigate patients further.

Methods At a single tertiary centre, 643 patients were identified to have undergone cardiac magnetic resonance (CMR) with evidence of LBBB between Jan 2008 – June 2013. 35 patients were identified to have normal left ventricular ejection fraction (LVEF) on echocardiograms performed within 12 months of the CMR at the same centre. Formally reported CMRs were retrospectively evaluated for significant clinical findings.

Results The mean age was 64.8(SD 10.8) years, 25 (71.4%) were female. 23 (63.9%) were found to have entirely normal CMRs. The LVEF was normal in 28 (80.0%) and impaired in 7 (20.0%) (mild n=5; moderate n=2). Severe valvular disease was found in three patients which had been underestimated on echocardiography (mitral regurgitation n=2; aortic regurgitation n=1). Myocardial stress imaging was performed in 27 (77.8%), of which only one patient had reversible ischaemia. Left ventricular hypertrophy was present in 6 (16.7%) and late gadolinium enhancement occurred in 5 (13.9%) (midwall n=3; epicardial=2; right ventricular insertion point n=1; subendocardial n=1). Four patients were reported to have LBBB-mediated cardiomyopathy, two patients had features suggestive of hypertrophic cardiomyopathy and one with established myocardial infarction.

Conclusion CMR detects significant pathology in up 40% of patients with LBBB and a normal echocardiogram.